The present invention relates to a system for generating isotopes useful for medical purposes, such as Mo-99, I-131, Xe-133, Y-90, Cs-137, I-125, and others, and in particular to a system employing a dry, granulated, uranium compound.
Medical isotopes are employed in nuclear medicine where they may be administered to a patient in a form that localizes to specific organs or cellular receptors where they may be imaged with special equipment. Medical isotopes may also be used in the treatment of disease exploiting the tissue-destructive power of short-range ionizing radiation after such localization.
Today, most radioisotopes used in nuclear medicine are produced in nuclear reactors employing highly enriched uranium (HEU). The reactors used for the production of Mo-99 for the United States are outside of the United States requiring the export of HEU and an attendant risk of nuclear proliferation associated with such out-of-country shipments.
It has been proposed to generate medical isotopes using low enriched uranium (LEU) which cannot be used directly to manufacture nuclear weapons. Systems for this purpose are described in US patent applications: 2011/0096887 entitled: “Device and Method for Producing Medical Isotopes” and 2010/0284502 entitled: “High Energy Proton or Neutron Source” hereby incorporated by reference.
In these systems, ions are directed through a target volume holding a gas to generate neutrons. The neutrons may expose a parent fissile material held in solution near the target volume in a fissile solution vessel. In one embodiment the target volume is annular and placed around a cylindrical fissile solution vessel holding the parent material solution. Ions are injected in a spiral through the target volume producing neutrons directed inwardly toward the parent material and outwardly toward a reflector.
Neutrons received in the neutron rich parent fissile material (such as LEU uranium) experience a multiplication in which neutrons striking the parent material generate additional neutrons which strike additional neutron rich material in a chain reaction. In a nuclear reactor, at steady power, the effective neutron multiplication factor (keff) is equal to 1. In a subcritical system, keff is less than 1. 
One problem with aqueous reactors is that it can be difficult to maintain a stable power level. This is because there exist strong feedback mechanisms in the neutron multiplication factor as the temperature of the fissile solution rises and as voids are generated (gas bubbles caused by radiolysis breaking water into hydrogen and oxygen). The rapid reduction in the neutron multiplication factor results in a decrease in power, which causes the neutron multiplication factor to increase again. In particular, a control system that is trying to maintain constant power in the reactor may not be able to react sufficiently fast to adequately control the system. The result is a system with an unstable power level and potential safety impacts.
Co-pending U.S. patent application Ser. No. 13/373,899 filed Dec. 5, 2011, hereby incorporated by reference in its entirety, describes an improved geometry for such aqueous reactors providing a fissile solution vessel in the form of a reduced thickness annulus. By controlling the aspect ratio of the annulus, improved reaction stability is employed and enhanced cooling provided.